Thin film transistors (TFTs) are known, and are of considerable commercial significance. For instance, amorphous silicon-based TFTs are used in a large fraction of active matrix liquid crystal displays.
TFFs with an organic active layer are also known. See, for instance, F. Garnier et al., Science, Vol. 265, pp. 1684-1686; H. Koezuka et at., Applied Physics Letters, Vol. 62 (15), pp. 1794-1796; H. Fuchigami et al., Applied Physics Letters, Vol. 63 (10), pp. 1372-1374; G. Horowitz et al., J. Applied Physics, Vol. 70 (1), pp. 469-475, and G. Horowitz et al., Synthetic Metals, vol. 41-43, pp. 1127-1130. These devices typically are field effect transistors (FETs). Such devices potentially have significant advantages over conventional TFTs, including a potentially simpler (and consequently cheaper) fabrication process, the possibility for low temperature processing, and compatibility with non-glass (e.g, plastic) substrates. Bipolar transistors that utilize both p-type and n-type organic material are also known. See, for instance, U.S. Pat. No. 5,315,129. S. Miyauchi et al., Synthetic Metals, 41-43 (1991), pp. 1155-1158, disclose a junction FET that comprises a layer of p-type polythiophene on n-type silicon.
However, despite considerable research and development effort, "organic" TFTs have not yet reached commercialization, at least in part due to the relatively poor device characteristics of prior art organic TFTs.
An important device characteristic of a switching transistor is the on/off ratio of the source/drain current. Prior art organic TFTs typically have relatively low on/off ratios. For instance, H. Fuchigami et al. (op. cit.) recently reported a device that had carrier mobility comparable to amorphous silicon, but had an on/off ratio of only about 20. That paper also discloses purification of semiconducting material (PTV) to reduce the carrier scattering by impurities. The material had a conductivity in the range 10.sup.-5 -10.sup.-6 S/cm.
H. Koezuka et al. (op. cit.) report attainment of an on/off ratio (modulation ratio) of the channel current of about 10.sup.5 in a device with doped polypyrole (a highly conducting polymer)-coated source and drain contacts. According to these authors, this is the highest on/off ratio achieved in organic FETs. Nevertheless, the reported on/off ratio is still substantially smaller than on/off ratios typically available in conventional FETs and demanded for many potential applications of organic TFTs. Furthermore, the organic TFT had very low carrier mobility (2.times.10.sup.-4 cm.sup.2 /V.s), and thus would not have been suitable for high-speed operation.
In view of the potential significance of organic TFTs, it would be desirable to have available such devices that have improved characteristics, including improved on/off ratio of the source/drain current. This application discloses such devices, and a method of making the devices.
Definitions and Glossary
An "organic semiconductor" herein is a material that contains a substantial amount of carbon in combination with other elements, or that comprises an allotrope of elemental carbon (excluding diamond), and exhibits charge carrier mobility of at least 10.sup.-3 cm.sup.2 /V.s at room temperature (20.degree. C.). Organic semiconductors of interest for TFTs typically have conductivity less than about 1 S/cm at 20.degree. C.
A "p-type" ("n-type") organic semiconductor herein is an organic semiconductor in which the Fermi energy is closer to (farther from) the energy of the highest occupied orbital of the molecules or aggregates present in the material than it is to (from) the energy of the lowest unoccupied orbital. The term is also intended to mean an organic semiconductor which transports positive charge carriers more (less) efficiently than negative carriers. Positive (negative) carriers are generally referred to as "holes" ("electrons").